1. Field of the Invention
This invention relates to a radiation image reproducing method for a radiation image recording and reproducing system in which a stimulable phosphor sheet is used for storing a radiation image information therein and the stimulable phosphor sheet is exposed to stimulating rays to emit light in proportion to the stored radiation energy and then the emitted light is detected and converted to electric image signals for displaying or reproducing a visible radiation image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet comprising the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays which cause it to emit light in proportion to the radiation energy stored. The light emitted from the stimulable phosphor sheet when the sheet is exposed to the stimulating rays is photoelectrically detected and converted into an electric image signal, which is processed as desired, to reproduce a visible image having improved quality, particularly high diagnostic efficiency and accuracy. The visible image which is finally obtained may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT).
In such a radiation image recording and reproducing system, a read-out apparatus as shown in FIG. 2 is proposed to be used for scanning the stimulable phosphor sheet with stimulating rays as of a laser beam to have light emitted from the stimulable phosphor sheet and reading out the emitted light photoelectrically.
Referring to FIG. 2, a stimulating light beam 2 is emitted from a stimulating light source 1 and the diameter of the light beam 2 is rigidly controlled by means of a beam expander 3, and is deflected by a light deflector 4 such as a galvanometer mirror to scan a stimulable phosphor sheet 10 by way of a mirror 5. Between the light deflector 4 and the mirror 5 is provided an f.theta.-lens 6 so that the light beam 2 may perform a main scanning in the direction of arrow C on the sheet 10 with a constant beam diameter at a constant speed. The sheet 10 is fed in the direction of arrow A to perform a sub-scanning, whereby the stimulating light beam scans the whole face of the sheet 10. By scanning the stimulating light beam 2 on the sheet 10, the sheet 10 emits light of such amount as to correspond to the stored radiation energy and the emitted light enters a light guide (light guiding sheet) 8 through the entrance face 8a thereof. The emitted light having entered the light guide 8 is transmitted to the exit face 8b by repeating total reflection and is received by a photodetector 9 such as a photomultiplier tube which is provided adjacent to the exit face 8b of the light guide. The light guide 8 and the photodetector 9 constitute a photoelectric read-out means 7.
The emitted light detected by the photodetector 9 is converted into an electric image signal and output from the photodetector 9, which signal is in turn amplified, A/D-converted, signal-processed and output as a visible image on a recording material such as a photosensitive material or a display device such as a CRT.
In the embodiment shown in FIG. 2, a reflector 14 is provided in front of the entrance face 8a of the light guide 8 with the scanning line located therebetween to reflect effectively the light emitted from the sheet 10 toward the entrance face 8a of the light guide 8 and increase the light collecting efficiency of the read-out system.
In the above-described read-out system, however, there occurs a so-called flare phenomenon so that, as shown in FIG. 3 when viewed from the direction of arrow B in FIG. 2, the stimulating light beam 2a impinging upon the sheet 10 is reflected by the surface of the sheet 10 and the reflected stimulating light (scattering light) 2b is reflected again by various parts of the read-out apparatus such as the reflector 14, the entrance face 8b of the light guide 8 and so forth and then impinge upon the sheet 10 again causing light emitted again therefrom. The extra light emitted again from the sheet 10 by this phenomenon is not the light from the point scanned originally by the stimulating light beam 2a, and accordingly, it will be a noise in the finally obtained radiation image and deteriorate the quality of the image lowering the contrast of the image.